Technical Field
The present invention relates to a radiation imaging system, particularly to a photon count-based X-ray imaging system, and also to a method for achieving X-ray imaging by the system and key devices thereof, which belong to the field of medical image technologies.
Related Art
The penetrating power of X-rays is particularly strong. When the X-rays pass through a sample made up of light elements such as carbon, hydrogen and oxygen, they leave no observable traces like visible light penetrates glass. This is very adverse for medical diagnosis. For example, for the diagnosis of breast tumor, the breast tumor, in the early stage of the development, is still the focus composed of light elements, the absorption-contrast imaging is helpless, and it is visible to the absorption-contrast imaging until calcification is generated in the late stage of the development of the breast tumor. This has missed the best time to treatment, seriously affecting the patients' chances of recovery.
With continuous development of the X-ray imaging technology, it is found that phase information carried by the X-rays after penetrating the sample can also be used for imaging the internal structure of the sample, and the phase drift section of the X-rays is 100-1000 times higher than the absorption section; the internal structure of the sample can be observed by acquiring phase information and carrying out recovery. For weak absorption materials composed of light elements, the change of the phase of the X-rays is more evident that the change of light intensity. X-ray phase-contrast imaging can more easily detect the internal structure of the sample than the traditional absorption-contrast imaging.
After more than 30 years of development, the X-ray phase-contrast imaging technology mainly uses the following four methods:
(1) Crystal interference contrast-imaging method: A complete crystal is cut into three very thin 3L shapes with bases connected together, which are respectively a beam splitter, a projection crystal and an analysis crystal. An X-ray is incident and passes through the first crystal, and then is diffracted and separated into two beams of coherent light. One beam of light is used as reference light, and a phase changer is placed on a propagation path thereof to continuously change light. The method is relatively strict in demanding mechanical stability of the experiment device; as a final diffraction pattern is detected after the incident X-ray penetrates 3 layers of crystals, the photon utilization is low, and a strong light source or a longer exposure time is required to make up. As the crystal size is limited, the method is only suitable for some small-size samples, and is only applicable to synchronous radiation at present.
(2) Crystal diffraction enhancement method: after heterogeneous X-rays emitted by the X-ray source go through a complete crystal, X-rays of which the incident angle meets the bragg diffraction condition (that is, the condition of producing coherent light interference) can pass through a monochromatic crystal, thus forming monochromatic light. An analysis crystal is placed behind the sample to serve as an angle analyzer, followed by a detector to record images. After the monochromatic light penetrates the sample, the analysis crystal converts phase information to light intensity information. By using the analysis crystal and adjusting the angle of the analysis crystal, X-rays transmitted, refracted and small-angle scattered after passing through the sample are enhanced or weakened, and thus diffraction enhancement imaging has three mechanisms of producing contrast, which are respectively absorption contrast, refraction contrast and extinction contrast obtained by filtering small-angle scattering.
(3) Grating shearing method: the monochromatic light is used to irradiate a grating, and a periodic image may appear at a certain distance behind the grating, that is, the “Tablot-Lau effect”, as shown in FIG. 1. By use of the grating self-imaging effect and through design of a light path, an image of a first phase grating is matched with a second absorption grating, then moire fringes formed by the sample are analyzed, and wave fronts can be quantitatively recovered. At present, there are two implementation schemes for the method, one is producing a phase shift of π/2 and the other is producing a phase shift of π. The advantage of the method lies in no longer relying on synchrotron radiation light sources with high brightness and higher coherence, thus having an extensive application prospect.
The method (2) and the method (3) are X-ray phase-contrast imaging methods based on optical analysis elements. The function of such optical analysis elements is to generate phase differential images, thus improving the boundary contrast of the images, and quantitative phase recovery needs to be carried out through a certain experimental mechanism and a corresponding algorithm.
(4) Phase-contrast imaging method based on X-ray free propagation: the method is also referred to as an X-ray in-line phase-contrast imaging method, which, according to different light sources used, is divided into monochromatic X-ray in-line phase-contrast imaging and polychromatic X-ray in-line phase-contrast imaging. The polychromatic X-ray in-line is based on a light intensity propagation equation proposed by K. A. Nugent in the University of Melbourne Australia. The in-line method is simpler in implementation, as long as the focal point of the X-ray source is small enough, phase-contrast imaging can be achieved on a device based on absorption contrast, however, as a phase second derivative is obtained with the in-line method, it is relatively difficult in phase recovery.
For example, in a Chinese invention patent application with Application Number of 200410053014.6, an X-ray contrast imaging method and system are disclosed. According to the scheme, the sample is imaged on a detector through in-line outline imaging, a distance between a light source point generated by a microfocus X-ray source and a sample on a scanning stage is adjusted according to the sample, and a distance between the sample and the detector is adjusted at the same time. However, an image obtained according to the scheme is a phase second-order differential image of the sample, how phase recovery is carried out to get a phase map of the sample on the basis of the second-order differential image is not described, nor how slice reconstruction and 3D imaging of phase contrast are achieved is described. In addition, as the brightness of the microfocus X-ray source is very low, the detector takes a longer time to expose, and it is difficult to meet the actual needs of clinical applications.
For another example, in a Chinese invention patent application with Application Number of 200810166472.9, an X-ray grating phase-contrast imaging system and method are disclosed. According to the method, phase-contrast imaging under incoherent conditions of approximate decimeter order-of-magnitude field can be achieved by using an X-light machine, a multi-seam collimator such as a source grating, and two absorption gratings. However, in the technical solution, the making of the grating is still a bottleneck, which will restrict actual application of the grating phase-contrast imaging technology in medicine and industry.
In addition, the existing phase-contrast imaging systems mostly employ a detector based on energy integral, such that the radiation imaging system utilizes the X-rays penetrating the sample at a lower rate. On the other hand, although structural information inside the sample can be obtained by using the detector based on energy integral, the capability of acquiring matter composition information of the sample is insufficient.
In addition, a synchronous radiation X-ray source belongs to a large scientific device, the equipment and maintenance costs are expensive, using it as medical clinical diagnostic equipment is not in line with the principle of effective utilization of energy and resources, and the imaging diagnosis cost cannot be afforded by general patients.